Frank Breher

The reduction chemistry of ferrocenylborole.

In the chemistry of functionalized metallocenes, borylated ferrocenes have drawn great attention owing to their potential applications as electron sponges, anion chemosensors, and redox-active macromolecules. Electrochemical stimulation of the Fe/Fe couple can lead to significant changes in the molecular structure, and to the anion binding properties of the system. One distinct structural feature of borylated ferrocenes is the bending of the boryl group towards the iron atom. Both experimental and theoretical studies of these molecules reveal a direct through-space interaction between the filled iron 3d orbital and the empty 2p orbital at boron. This “dip” angle can be reduced by incorporation of p-donating substituents to the boron atom, coordination with Lewis bases, by increasing the number of boryl functionalities on the Cp rings, or oxidation of the ferrocene to ferrocenium. The structural changes induced by electronic reduction of the boryl group, however, have not received much attention. Investigations on the reduction chemistry of organoboranes have led to the isolation of various interesting boroncentered radical species. The reduction potential of the boron center can be tuned by substitution with fluorinated aryl groups, introduction of a second boryl moiety, attachment of cationic functionalities, or by incorporating the boron atom into an antiaromatic ring system. In line with our interests in antiaromatic boracycles and metallocene chemistry, we now report the reduction chemistry of 1-ferrocenylborole and the isolation of the resulting reduced species. In our previous work, the most striking structural feature of 1-ferrocenyl-2,3,4,5-tetraphenylborole (1) is the large dip angle of 29.48. This observation is attributed to the strong Fe–B interaction resulting from the antiaromatic nature of the borole moiety. The presence of electro-active ferrocene (Fc) and borole units in 1 prompted us to investigate its electrochemistry. The redox behavior of 1 in CH2Cl2 and in THF was studied by cyclic voltammetry (referenced against the Fc/Fc couple). Compound 1 displays one broad irreversible oxidation peak around Epa = 0.5 V in CH2Cl2. (see Figure S1 of the Supporting Information). The oxidation behavior of 1 is distinctly different from that observed for 9-ferrocenyl borafluorene, which shows a reversible Fe/Fe redox couple at 0.01 V (vs. Fc/Fc). The oxidation process of 1 is thus anodically shifted as a result of the stronger Fe–B interaction in 1. The oxidation event is also more positively shifted than that of the ferrocenylboron dication (E1/2 = 0.24 V), in which the effect on Fe/Fe couple is solely inductive. According to the study of 9-ferrocenyl borafluorene, one-electron oxidation results in the formation of a ferrocenium cation and a dip angle decrease from 25.58 to 6.38. However, the synthetic procedure for ferrocenium borafluorene is not applicable for the ferrocenium borole. This difference is because the transformation of a neutral borole to a cationic borole greatly enhances the electron deficiency and Lewis acidity of the boron center, and leads to undesired coordination and/or decomposition of the resulting cationic molecule. Most revealingly, two well-separated reduction waves for 1 were identified in THF solution. As shown in Figure 1, 1 displays a quasi-reversible reduction event centered at E1/2 = 1.96 V indicating the formation of stable borole radical anion, [1]C . Note that in contrast to the well-studied borole dianions, the paramagnetic 5p-electron radical anion of

Silver–Ethene Complexes [Ag(η2‐C2H4)n][Al(ORF)4] with n=1, 2, 3 (RF=Fluorine‐Substituted Group)

Compounds including the free or coordinated gas-phase cations [Ag(eta(2)-C(2)H(4))(n)](+) (n = 1-3) were stabilized with very weakly coordinating anions [A](-) (A = Al{OC(CH(3))(CF(3))(2)}(4), n = 1 (1); Al{OC(H)(CF(3))(2)}(4), n = 2 (3); Al{OC(CF(3))(3)}(4), n = 3 (5); {(F(3)C)(3)CO}(3)Al-F-Al{OC(CF(3))(3)}(3), n = 3 (6)). They were prepared by reaction of the respective silver(I) salts with stoichiometric amounts of ethene in CH(2)Cl(2) solution. As a reference we also prepared the isobutene complex [(Me(2)C=CH(2))Ag(Al{OC(CH(3))(CF(3))(2)}(4))] (2). The compounds were characterized by multinuclear solution-NMR, solid-state MAS-NMR, IR and Raman spectroscopy as well as by their single crystal X-ray structures. MAS-NMR spectroscopy shows that the [Ag(eta(2)-C(2)H(4))(3)](+) cation in its [Al{OC(CF(3))(3)}(4)](-) salt exhibits time-averaged D(3h)-symmetry and freely rotates around its principal z-axis in the solid state. All routine X-ray structures (2theta(max.) < 55 degrees) converged within the 3sigma limit at C=C double bond lengths that were shorter or similar to that of free ethene. In contrast, the respective Raman active C=C stretching modes indicated red-shifts of 38 to 45 cm(-1), suggesting a slight C=C bond elongation. This mismatch is owed to residual librational motion at 100 K, the temperature of the data collection, as well as the lack of high angular data owing to the anisotropic electron distribution in the ethene molecule. Therefore, a method for the extraction of the C=C distance in [M(C(2)H(4))] complexes from experimental Raman data was developed and meaningful C=C distances were obtained. These spectroscopic C=C distances compare well to newly collected X-ray data obtained at high resolution (2theta(max.) = 100 degrees) and low temperature (100 K). To complement the experimental data as well as to obtain further insight into bond formation, the complexes with up to three ligands were studied theoretically. The calculations were performed with DFT (BP86/TZVPP, PBE0/TZVPP), MP2/TZVPP and partly CCSD(T)/AUG-cc-pVTZ methods. In most cases several isomers were considered. Additionally, [M(C(2)H(4))(3)] (M = Cu(+), Ag(+), Au(+), Ni(0), Pd(0), Pt(0), Na(+)) were investigated with AIM theory to substantiate the preference for a planar conformation and to estimate the importance of sigma donation and pi back donation. Comparing the group 10 and 11 analogues, we find that the lack of pi back bonding in the group 11 cations is almost compensated by increased sigma donation.

Development of a Metal‐Ion‐Mediated Base Pair for Electron Transfer in DNA

A new C-nucleoside structurally based on the hydroxyquinoline ligand was synthesized that is able to form stable pairs in DNA in both the absence and the presence of metal ions. The interactions between the metal centers in adjacent Cu(II)-mediated base pairs in DNA were probed by electron paramagnetic resonance (EPR) spectroscopy. The metal-metal distance falls into the range of previously reported values. Fluorescence studies with a donor-DNA-acceptor system indicate that photoinduced charge-transfer processes across these metal-ion-mediated base pairs in DNA occur more efficiently than over natural base pairs.

Transfer of a disilenyl moiety to aromatic substrates and lateral functional group transformation in aryl disilenes.

The reaction of 1 equiv of the disilenide Tip2Si═Si(Tip)Li (5; Tip = 2,4,6-(i)Pr3C6H2) with para-substituted phenyl iodides, 4-X-PhI, transfers the Tip2Si═Si(Tip) moiety with elimination of lithium iodide to yield the laterally functionalized disilenes Tip2Si═Si(Tip)(4-X-Ph) [X = H (6a), F (6b), Cl (6c), Br (6d), I (6e)]. The UV-vis absorptions of 6a-d suggest a linear correlation with electronic Hammett parameters. In addition, X-ray structural analyses of 6a-d verified the theoretically predicted linear dependence of the Si═Si bond length and trans-bent angles. The p-bromophenyl-substituted disilene 6d undergoes a metal-halogen exchange reaction to give 6f (X = Li), which was trapped with Me3SiCl to afford 6g (X = SiMe3). In the case of simple phenyl halides PhX without additional functionality, the reaction with 5 proceeded smoothly for X = Br, but phenyl chlorides and fluorides did not react at room temperature even after one week, hinting at an S(N)2-type aromatic substitution mechanism. Reactions of p- and m-diiodobenzene with 5 afford the corresponding phenylene-bridged tetrasiladienes p-7 and m-7. While red p-7 (λ(max) = 508 nm) exhibits efficient conjugation of the two Si═Si bonds with the phenylene linker, the conjugation in yellow m-7 (λ(max) = 449 nm) is much less effective. Electrochemical studies of m-7 and p-7 as well as density functional theory calculations and electron paramagnetic resonance studies of their respective radical anions provided further support for the notion of conjugation.

Coinage metal complexes of tris(pyrazolyl)methanide [C(3,5-Me2pz)3]−: κ3-coordination vs. backbone functionalisation

Tris(pyrazolyl)methanides, [C(3,5-R2pz)3]-, contain an unassociated tetrahedral carbanionic centre in the bridgehead position. In addition to nitrogen donor centres for transition metal coordination, an accessible reactive site for further manipulations is available in the backbone of the ligand. The coordination variability of the ambidental C-/N ligand [C(3,5-Me2pz)3]- was elucidated by investigating its coinage metal complexes. Two principle coordination modes were found for complexes of general formula [LMPR3] (with M = Cu(I), Ag(I), Au(I); L =[C(3,5-Me2pz)3]-; R = Ph, OMe). While for Cu(I) (2,3) and Ag(I) (4) complexes the anionic ligand acts as a face-capping, six electron N3-donor, gold(I) (5) is coordinated by the bridging carbanion yielding a two coordinate Au(I) complex comprising a covalent Au-C bond. The complexes featuring the kappa3-coordinated N3-donor ligand were investigated by 31P CP (MAS) NMR in the solid state.

[2.2]Paracyclophane derived bisphosphines for the activation of hydrogen by FLPs: application in domino hydrosilylation/hydrogenation of enones

The heterolytic splitting of hydrogen by two types of [2.2]paracyclophane derived bisphosphines (1, 2a and 2b) in combination with tris(pentafluorophenyl)borane (3) at room temperature is described. The corresponding frustrated Lewis pairs (FLPs) exhibit different behavior in the activation of hydrogen. This results from diverse steric and electronic properties of the bisphosphines. The reactivity of the frustrated Lewis pairs was exploited in the first diastereoselective domino hydrosilylation/hydrogenation reaction catalyzed by FLPs.

Pentacarba‐arachno‐tridecaalane (AlMe)8(CCH2Ph)5H with an Al8C5 Skeleton—The First Polyhedral Carbaalane

Hydroalumination of an aluminum alkynide led to the formation of a polyhedral carbaalane as the first example of a new class of aluminum derivatives analogous to carbaboranes. A pentacarbatridecaalane was obtained, which has an Al8 C5 skeleton and which, owing to the number of electrons in the cluster, belongs to the arachno-type group of compounds (see structure).

Pentagerma[1.1.1]propellane: A Combined Experimental and Quantum Chemical Study on the Nature of the Interactions between the Bridgehead Atoms

Ge, whiz! A detailed study of the synthesis, structure, redox chemistry, and bonding properties of pentagerma[1.1.1]propellane (1, see picture) examines fundamental aspects of the interactions between the bridgehead germanium atoms. DFT and CASSCF calculations unravel the biradicaloid characteristics of 1, and preliminary reactivity studies indicate that 1 features some radical-type behavior.

Experimental charge density distribution of non-coordinating sp3 carbanions in [Mg{(pz*)3C}2]

In this communication we present the experimental charge density distribution in [Mg{(pz*)(3)C}(2)] (1), (pz* = 3,5-dimethylpyrazolyl), containing two non-coordinating sp(3) carbanionic lone-pairs.

The hydroalumination of diethynes with di(tert-butyl)aluminium hydride: a facile method for the synthesis of organometallic dialuminium compounds

Abstract Di(tert-butyl)aluminium hydride reacted with the diynes 1,4-bis(trimethylsilyl)-1,3-butadiyne and 1,4-bis(trimethylsilylethynyl)benzene by hydroalumination and addition of one Al–H bond to each CC triple bond. A selective cis addition was observed in both cases to yield alkenes with Z configuration exclusively. The aluminium atoms were attached to those carbon atoms which bear the trimethylsilyl groups. While the butadiyne product (1) decomposed upon heating in benzene solution, the diene (2), obtained by the twofold hydroalumination of 1,4-bis(trimethylsilylethynyl)benzene, showed a remarkable rearrangement and gave the thermodynamically favoured product (4) almost quantitatively, which has two alkenes in E configuration. All products were characterized by crystal structure determinations.